This invention relates generally to static electric power conversion apparatus of the inverter type, and more particularly it relates to improved control means for such apparatus capable of "driving" adjustable speed a-c motors.
In many applications of electric motors, alternating current induction motors are preferred to other kinds because of their relative simplicity, compactness, ruggedness, ease of maintenance, light weight, and low cost. The torque developed by an induction motor can be shown to be a function of both the amplitude-to-frequency ratio of the sinusoidal alternating voltage applied to its stator windings and the slip speed of the motor (i.e., the difference between the actual speed of the rotor surface and the angular speed of the stator flux wave, both expressed in terms of radians per second). Where such a motor is required to run at variable speeds, it is common practice to supply its windings with a-c power which is so conditioned that the frequency as well as the amplitude of the stator voltage are adjustable as desired. By suitably controlling these two parameters, the motor can be caused to operate, for example, with constant load torque for speeds between zero and a given corner point speed (i.e., the highest speed at which the motor flux can be held constant) and with reduced torque but substantially constant horsepower for a range of speed variations above the corner point speed, which characteristic is desirable in certain applications such as traction drives for electrically propelled rail vehicles.
Polyphase alternating voltage of variable amplitude and frequency can be advantageously derived from a given d-c power source, or from a fixed frequency a-c source, by using static electric power apparatus in which a plurality of pairs of alternately conducting controllable electric valves are interconnected and arranged so as to convert the voltage applied to the input terminals of the apparatus into polyphase output voltages suitable for energizing the stator windings of a 3-phase, adjustable speed electric motor. Typically the valves comprise unidirectionally conducting switching elements of the kind having the ability to hold off forward voltage until turned "on" in response to a suitable control or gate signal. One family of such elements is generally known by the names "controlled rectifier" or "thyristor," and I prefer to use this family in the present invention. Once triggered or "fired" by its control signal, a thyristor switches from a blocking or non-conducting state to a forward conducting state in which it can freely conduct motor current until this current is subsequently extinguished by the commutating action of external circuit components. A free-wheeling diode can be connected in inverse parallel relationship with each of the load current conducting thyristors in order to conduct motor current during intervals when the thyristor is reverse biased (i.e., anode potential is negative with respect to cathode).
There are many different circuit configurations and operating modes for power conversion apparatus wherein thyristors are used as the main switching elements. Such apparatus conventionally includes suitable firing and commutating means for periodically reversing or switching the conducting states of the respective thyristors in each of the alternately conducting pairs. By repeating this switching action for each thyristor pair in a predetermined cycle pattern and by staggering the patterns of the thyristor pairs associated with the respective phases of the motor, the desired 3-phase alternating voltages are developed at the output terminals of the apparatus. The frequency of the fundamental component of the output voltage waveform is determined by the frequency of the cyclic pattern of switching the thyristor pairs. The amplitude of the output voltage can be linearly varied with frequency either by correspondly varying the voltage applied to the input terminals of the conversion apparatus or, assuming that the magnitude of the input voltage is constant, by appropriately controlling the operation of the firing and commutating means in the apparatus itself. One of the most advantageous means of controlling the output voltage within the conversion apparatus is to utilize a switching time-ratio control technique.
One very effective system of the switching time-ratio type is known as multiple pulse width modulation (PWM). In this system the conducting states of each pair of alternately conducting thyristors are switched more than twice each half cycle of the motor voltage, thereby chopping the half cycle waveform into a series of discrete, relatively narrow pulses of alternately differing (e.g., positive and negative) potentials. The time durations or widths of the individual pulses and the number of pulses per half cycle of fundamental frequency are varied in accordance with a preselected control strategy so as to vary the average voltage applied to the motor, thereby varying the amplitude of the fundamental sinusoidal component of the motor terminal voltage as desired. The control strategy is preferably selected so that during each half cycle the train of rectangular output voltage pulses is modulated sinusoidally, and for this purpose a triangle interception mode of PWM has heretofore been proposed.
In the triangle interception mode of PWM, a sine wave reference signal (also referred to as the "modulation wave") of variable amplitude and frequency is compared with a triangular timing waveform (also referred to as the "carrier wave") having a constant amplitude and a frequency which is appreciably higher than that of the reference signal, and the conducting states of a thyristor pair are switched each time the timing waveform intercepts the reference signal. As a result, the fundamental component of the voltage at the associated output terminal has the same frequency and is approximately in phase with the sine wave reference signal, its amplitude is a linear function of the modulation ratio (i.e., the ratio of the reference signal amplitude to the timing waveform amplitude), and the number of pulses per half cycle of fundamental frequency is determined by the chopping ratio (i.e., the ratio of the frequency of the timing waveform to the frequency to the reference signal). So long as the chopping ratio is relatively high (e.g., greater than six), the harmonic distortion of the output voltage waveform is relatively low and the residual harmonics all have such a high order that they are virtually without influence on the average motor torque. Consequently the triangle interception technique of PWM avoids unacceptably large torque pulsations and harmonic losses in the motor when the adjustable speed drive is operating at speeds near zero. An example of such a scheme adapted for wide speed range motor drives is disclosed in Siemens-Zeitschrift 45 (1971) Heft 3, pages 154-61, "Pulswechselrichter zur Drehzahlsteuerung von Asynchronmaschinen" von Heintz, Tappeiner, und Weidelzahl. To avoid unwanted subharmonic voltage components or low frequency "beats" as the amplitude and frequency of the fundamental output voltage increase, it is common practice to synchronize the triangular timing waveform to the sinusoidal reference signal. But then it is necessary to increase the chopping ratio at the low end of the speed range in order to keep the frequency of the timing waveform sufficiently high to avoid excessive ripple current in the motor.
To obtain the maximum possible output voltage from any PWM conversion apparatus, the mode of operation must be changed to "square wave" wherein the thyristor pairs are switched only at half cycle intervals and all chops in between are dropped or omitted, whereby unmodulated square-wave voltages of fundamental frequency are applied to the respective motor terminals. This mode of operation results in the familiar 6-step voltage waveform across each of the stator windings of the adjustable speed, 3-phase induction motor (which windings conventionally are interconnected in a 3-wire star configuration so as to cancel third harmonics and multiples thereof throughout the whole speed range). Although the maximum output voltage waveform is known to contain 20% fifth harmonics and lesser percentages of seventh and higher harmonics, there are no perceptible torque pulsations because, with the adjustable speed drive now running at maximum voltage and relatively high fundamental frequency, the mechanical load driven by the motor and the motor rotor itself will have sufficient inertia to provide a smoothing effect. Operating in the square wave mode is desirable because it results in lower converter losses and permits reduction in equipment size.
In transitioning between the triangle interception PWM and the unmodulated square wave modes of operation, a problem exists because the interval between consecutive switching moments of a thyristor pair has a finite minimum limit (typically 100 to 300 microseconds) to allow time for successful commutation, and therefore the width of an output voltage pulse cannot be gradually varied between this limit and zero. Whenever a minimum-width pulse is dropped from or added to the output voltage waveform, a discontinuity occurs in the amplitude-to-frequency ratio of the output voltage and the motor is subject to a discrete surge of torque which may be objectionably large unless the change happened to occur in the vicinity of a zero crossing of the fundamental component of the output voltage. Also a phase change can occur which causes misalignment of inverter voltage with motor back emf and thus causes undesirable surge currents.
Others have previously suggested changing the modulation strategy of a PWM converter in a selected intermediate portion of the fundamental frequency range, which portion is disposed between the low end of the range where the basic triangle interception technique is effective and the high end of the range where the converter is operating in a square wave mode. Abbondanti and Nordby, in their paper entitled "Pulse Width Modulated Inverter Motor Drives with Improved Modulation" presented in October 1974 at the Ninth Annual Meeting of the IEEE Industry Applications Society in Pittsburgh, Pennsylvania (IEEE Conference Record 74CH)833-41A, pages 998-1006), disclose a series of transitional PWM modes wherein the timing waveform is synchronized to the reference signal, its frequency or slope is variously modified, and/or the amplitude of the reference signal is varied as necessary to reduce the number of chops to zero without discontinuity in the amplitude of the fundamental output waveform. Heintze et al, in their 1971 publication previously cited (see FIGS. 6 and 7), suggest replacing the triangle interception technique of PWM with an equivalent d-c level set method which automatically ensures both steady state and transient synchronization of the chops with the reference signal.
In the d-c level set mode of PWM, a sine wave reference signal (modulation wave) is compared with one or more voltage levels, and the conducting states of a thyristor pair are switched each time the reference signal crosses zero and each time its instantaneous magnitude equals a voltage level. By using this technique, the minimum-width pulses that are dropped from or inserted in the output voltage as its fundamental amplitude is changed are always the ones closest to the zero crossings of the fundamental waveform, and their effect on the effective value of the fundamental is therefore negligible.